Although cellular phone systems have been widespread in recent years, it has two major issues. One is a matter of data communication charges. This is a problem in that, since the cellular phone system is designed assuming speech service, efficient transmission of data is difficult by contrast, resulting in difficulty of reducing data communication charges.
Accordingly, it is difficult to cope with exploding demands of data communication caused by widespread use of the Internet.
The second issue is a problem of establishing an infrastructure. The more the cellular phone system is requested to provide advanced service at higher speed, the more the system becomes complex, so that it requires a lot of costs for the infrastructure. This results in problems in that communication charges increase, expansion of a service area takes a long time, and an area which is not provided with service appears.
In addition to the widespread use of the cellular phone system, a so-called wireless LAN attracts attention in accordance with increased demands of wireless data communication in the recent years. Although a wireless LAN has begun to come into widespread use as an indoor, independent wireless communication system, it has recently begun to become used as an outdoor public communication system.
Since a wireless LAN is designed for dedicated use for data communication, it is an optimal system for use in data communication. A strong demand is expected that the wireless LAN, which has such advantages, be used not only in data communication but also in audio communication which has been provided in conventional cellular telephone.
Nevertheless, a wireless data communication as typified by a wireless LAN has two problems in that it does not have a calling function and that it does not have a power saving function.
The first problem is caused by an inability of a network side to grasp the geographical position of a terminal (what wireless communication base station has an area in which the terminal is positioned) since the system is simplified. Accordingly, in the case of calling the terminal, a call signal must be transmitted from each of all wireless communication base stations.
However, this method has not been implemented to date because it wastes frequency resources and also releases, to the network, a large amount of control information associated with calling. In other words, prior art has a problem in that it is impossible that, by using a wireless communication system originally having no calling function, an audio communication service or the like that needs a calling function be provided.
The second problem is one caused by a wireless communication method for a wireless data communication system. In a wireless data communication system, in general, data is divided into units called packets, and a terminal and a wireless communication base station transmit information to each other in units of packets.
In this case, to enhance a packet multiplexing effect as much as possible, in general, for example, a method called CSMA (Carrier Sense Multiple Access) classified into a random access method is used.
In this method, one terminal sends its packets after confirming that another terminal or a base station sends no packets. In other words, the terminal cannot transmit packets unless it is always in a state capable of receiving packets. Accordingly, regardless of whether the terminal transmits packets, it must always operate only a receiving function, so that the power consumption of its circuit is very large. For example, a portable Internet wireless telephone terminal that is a combination of an existing wireless LAN unit and a small battery has a standby time of 24 hours or less. Accordingly, it may not be actually said that the portable Internet wireless telephone terminal does not serve for practical use.
This problem of power saving is not limited to a wireless communication system as typified by a wireless LAN, but applies to also wireless telephone systems such as cellular phones and PHSs. In these systems, a mechanism for a terminal to periodically and intermittently receive a broadcasting signal from a base station is utilized in order that, in a standby mode, the terminal may determine anytime whether it is called by a network side. Although this method achieves an extended standby time by reducing the amount of power consumption in the standby mode, more power saving and extension in standby time are expected.